This invention relates to a mass flow rate/density/viscosity sensor working on the Coriolis principle—herein-after referred to as a Coriolis sensor for short—and comprising two bent measuring tubes.
With such Coriolis sensors, whose measuring tubes, as is weil known, are set into vibration, particularly into flexural vibration with or without superposed torsional vibration, it is possible to measure not only the instantaneous mass flow rate of a fluid flowing in a pipe, but also the density of the fluid via the instantaneous vibration frequency of the measuring tubes and the viscosity of the fluid via the power required to maintain the vibrations of the tubes.
Since the temperature of the fluid is not constant during operation of the Coriolis sensor, and the density of the fluid, as is well known, is temperature-dependent, the Coriolis sensor is commonly provided with at least one temperature sensor for measuring the temperature of the fluid. For all those measurements, the Coriolis sensor is connected into the pipe in a pressure-tight manner and generally permanently, for example via flanges.
U.S. Pat. No. 4,187,721 discloses a Coriolis mass flow rate/density sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising:                a single, U-shaped measuring tube bent in one plane symmetrically with respect to an axis of symmetry, which                    is of one-piece construction and            has a straight inlet portion fixed in a support angle,            a straight outlet portion fixed in the support angle,            an offset inlet transition portion connected with the inlet portion,            an offset outlet transition portion connected with the outlet portion,            a first bent portion connected with the inlet transition portion,            a second bent portion connected with the outlet transition portion,            a straight base portion connecting the first and second bent portions;                        an excitation system                    which in operation causes the measuring tube together with an exciter carrier to vibrate as a tuning fork,            a first portion of which is fixed to the base portion in the area of the axis of symmetry, and            a second portion of which is fixed to the exciter carrier;                        a first optical sensor,                    a first portion of which is fixed to the measuring tube at a location                            where the inlet transition portion passes into the first bent portion, and                                    a second portion of which is fixed to the support angle; and                        a second optical sensor,                    a first portion of which is fixed to the measuring tube at a location                            where the outlet transition portion passes into the second bent portion, and                                    a second portion of which is fixed to the support angle.                        
JP-A 56-125 622 discloses a Coriolis mass flow rate sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising:                an omega-shaped measuring tube bent in one plane symmetrically with respect to an axis of symmetry which                    is of one-piece construction and            has a straight inlet portion with an inlet axis lying in said plane,            a straight outlet portion with an outlet axis aligned with the inlet axis,            an S-shaped inlet bend connected with the inlet portion,            an S-shaped outlet bend connected with the outlet portion, and            a vertex bend connecting the inlet and outlet bends;                        an excitation system                    which in operation causes the measuring tube together with an exciter carrier to vibrate as a tuning fork,            a first portion of which is fixed to the vertex bend in the area of the axis of symmetry, and            a second portion of which is fixed to the exciter carrier;                        a bar-shaped sensor carrier                    which extends perpendicular to the axis of symmetry,            a first end of which is fixed to the measuring tube at a location where the inlet bend passes into the vertex bend, and            a second end of which is fixed to the measuring tube at a location where the outlet bend passes into the vertex bend; and                        a strain-gage bridge disposed as a sensor arrangement on the sensor carrier.        
U.S. Pat. No. 4,127,028 discloses a Coriolis mass flow rate sensor designed to be installed in a-pipe through which a fluid flows at least temporarily, and comprising:                a first U-shaped measuring tube bent in a first plane symmetrically with respect to a first axis of symmetry;        a second U-shaped measuring tube bent in a second plane symmetrically with respect to a second axis of symmetry,                    which measuring tubes are arranged parallel to each other, are of one-piece construction, and are connected in series in terms of fluid flow, and            each of which measuring tubes has                            a straight inlet portion,                a straight outlet portion,                an S-shaped inlet bend connected with the inlet portion,                an S-shaped outlet bend connected with the outlet portion,                a first straight tube portion connected with the inlet bend,                a second straight tube portion connected with the outlet bend, and                a semicircular base bend connected with the first and second straight tube portions,                                    which inlet and outlet portions extend through a fixed member,                                                with the distance between the inlet and outlet portions of each measuring tube being less than the distance between the first and second straight tube portions of the respective measuring tube;                                                an excitation system                    which during operation causes the measuring tubes to vibrate as a tuning fork,            a first portion of which is fixed to the semicircular base bend of the first measuring tube in the area of the axis of symmetry of the first measuring tube, and            a second portion of which is fixed to the semicircular base bend of the second measuring tube in the area of the axis of symmetry of the second measuring tube;                        a first optical sensor,                    a first portion of which is fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations                            where the respective first straight tube portion passes into the respective semicircular base bend; and                                                a second optical sensor,                    a first portion of which is fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations                            where the respective second straight tube portion passes into the respective semicircular base bend.                                                
U.S. Pat. No. 4,622,858 discloses a Coriolis mass flow rate sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising:                a first straight measuring tube;        a second straight measuring tube,                    which measuring tubes are arranged parallel to each other,            are of one-piece construction, and            are connected in parallel in terms of fluid flow by means of an inlet manifold and an outlet manifold;                        a driving mechanism                    which in operation vibrates the measuring tubes as a tuning fork,            a first portion of which is fixed to the first measuring tube midway between the inlet manifold and the outlet manifold, and            a second portion of which is fixed to the second measuring tube midway between the inlet manifold and the outlet manifold;                        a first electrodynamic sensor,                    a first portion of which is fixed to the first measuring tube midway between the driving mechanism and the inlet manifold, and a second portion of which is fixed to the second measuring tube midway between the driving mechanism and the inlet manifold; and                        a second electrodynamic sensor,                    a first portion of which is fixed to the first measuring tube midway between the driving mechanism and the outlet manifold, and a second portion of which is fixed to the second measuring tube midway between the driving mechanism and the outlet manifold.                        
U.S. Pat. No. 6,006,609 discloses a Coriolis mass flow rate/density/viscosity sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising:                a single straight measuring tube of one-piece construction                    which is provided with a cantilever at its midpoint, and            an inlet end and an outlet end of which are mounted in a support frame which is disposed in a housing;                        an excitation arrangement                    which in operation sets the measuring tube into flexural vibrations and into torsional vibrations equal in frequency to the flexural vibrations, and            first portions of which are fixed to the cantilever and second portions of which are fixed to the support frame;                        a first sensor,                    a first and a second portion of which are fixed to the measuring tube and the support frame, respectively, approximately midway between the inlet end and the cantilever; and                        a second sensor,                    a first and a second portion of which are fixed to the measuring tube and the support frame, respectively, approximately midway between the outlet end and the cantilever.                        
U.S. Pat. No. 5,796,011, particularly in connection with FIG. 5, describes a Coriolis mass flow rate sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising:                a first measuring tube bent in a first plane symmetrically with respect to a first axis of symmetry;        a second measuring tube bent in a second plane symmetrically with respect to a second axis of symmetry,                    which measuring tubes are arranged parallel to each other and are of one-piece construction, and            each of which measuring tubes has                            a straight inlet portion with an inlet axis lying in the first plane and the second plane, respectively,                a straight outlet portion with an outlet axis aligned with the inlet axis,                an inlet bend connected with the inlet portion,                an outlet bend connected with the outlet portion, and                a circular-arc-shaped vertex portion of minimum height connected with the inlet bend and outlet bend,                                    which inlet portions and which outlet portions are connected in parallel in terms of fluid flow by means of an inlet manifold and an outlet manifold, respectively, and                    which manifolds are mounted in a support frame which forms part of a housing;                                                                                a first node plate rigidly connecting the two measuring tubes at a location                    where the inlet bend passes into die circular-arc-shaped vertex bend;                        a second node plate rigidly connecting the two measuring tubes at a location                    where the outlet bend passes into the circular-arc-shaped vertex bend;                        an excitation system                    which in operation causes the measuring tubes to vibrate as a tuning fork,            a first portion of which Is fixed to the circulararc-shaped vertex bend of the first measuring tube in the area of the axis of symmetry of the first measuring tube, and            a second portion of which is fixed to the circular-arc-shaped vertex bend of the second measuring tube in the area of the axis of symmetry of the second measuring tube;                        a first sensor,                    a first portion of which is fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations                            where the respective inlet bend passes into the respective circular-arc-shaped vertex bend;                                                a second sensor,                    a first portion of which fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations                            where the respective outlet bend passes into the respective circular-arc-shaped vertex bend;                                                a feedthrough mounted in the support frame opposite the circular-arc-shaped vertex bends and containing several electric conductors; and        a printed-circuit board attached to the support frame and extending between the inlet manifold and outlet manifold and having conducting tracks                    via which leads of the excitation system and the sensors are connected to the conductors of the feedthrough.                        
To the above referred ensembles of features of the individual prior-art arrangements it should be added that a straight measuring tube or straight measuring tubes are preferably made of pure titanium, a high-titanium alloy, pure zirconium, or a high-zirconium alloy, since, compared with measuring tubes of stainless steel, which is suitable material for straight measuring tubes in principle, shorter overall lengths are obtained, and that a bent measuring tube or bent measuring tubes are preferably made of stainless steel, although titanium or zirconium or their alloys are suitable materials for such tubes as well.
The design principle of the Coriolis mass flow rate sensor according to U.S. Pat. No. 5,796,011 permits the use of only such circular-arc vertex bends which have a great radius of curvature, i.e., where the distance between the circular-arc vertex bend and the inlet/outlet axis is minimal as a function of the inside diameter and the wall thickness of the measuring tubes and of a permissible, temperature-range-induced mechanical stress. For distances between the vertex and the inlet/outlet axis that are greater than the minimum distance, however, particularly for distances greater than the minimum distance by an order of magnitude, the design principle of U.S. Pat. No. 5,796,011 is unsuitable.
Therefore, starting from the design principle U.S. Pat. No. 5,796,011, it is an object of the invention to provide a Coriolis mass flow rate/density/viscosity sensor in which the distance between the vertex of the vertex bend and the inlet/outlet axis can be virtually arbitrarily great. At the same time, high measurement accuracy, for example of the order of ±0.5%, is to be achievable, manufacturing costs are to be minimized as compared to those of prior-art mass flow rate sensors, mass flow rate/density sensors, or mass flow rate/density/viscosity sensors, and a shorter overall length is to be made possible.
To attain these objects, the invention provides a Coriolis mass flow rate/density/viscosity sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising:                a first measuring tube bent to a V shape in a first plane symmetrically with respect to a first axis of symmetry;        a second measuring tube bent to a V shape in a second plane symmetrically with respect to a second axis of symmetry,                    which measuring tubes are arranged parallel to each other and are each of one-piece construction, and            each of which measuring tubes has                            a straight inlet portion with an inlet axis lying in the first plane and second plane, respectively,                a straight outlet portion with an outlet axis lying in the first plane and second plane, respectively, and aligned with the inlet axis,                an inlet bend connected with the inlet portion,                an outlet bend connected with the outlet portion,                a first straight tube portion connected with the inlet bend,                a second straight tube portion connected with the outlet bend, and                a vertex bend connected with the first and second straight tube portions,                                    which inlet portions are fixed in an inlet manifold, which outlet portions are fixed in an outlet manifold, and                    which manifolds are mounted in a support frame which forms part of a housing;                                                                                an excitation arrangement                    which in operation causes the measuring tubes to vibrate as a tuning fork,            a first portion of which is fixed to the vertex bend of the first measuring tube in the area of the axis of symmetry of the first measuring tube, and            a second portion of which is fixed to the vertex bend of the second measuring tube in the area of the axis of symmetry of the second measuring tube;                        a first velocity or displacement sensor,                    a first portion of which is fixed to the first straight tube portion of the first measuring tube, and            a second portion of which is fixed to the first straight tube portion of the second measuring tube;                        a second velocity or displacement sensor, positioned symmetrically with respect to the axes of symmetry of the measuring tubes,                    a first portion of which is fixed to the second straight tube portion of the first measuring tube, and a second portion of which is fixed to the second straight tube portion of the second measuring tube;                        a feedthrough mounted in the support frame opposite the vertex bends and containing several electric conductors; and        a printed-circuit board attached to the support frame and extending between the support frame and the vertex bends and having conducting tracks                    to which leads of the excitation system and of the velocity or displacement sensors are connected.                        
In a preferred embodiment of the invention, the measuring tubes                are rigidly connected by a first node plate in the vicinity of a location                    where the respective inlet portion passes into the respective inlet bend,                        are rigidly connected by a second node plate in the vicinity of a location                    where the respective inlet bend passes into the respective first straight tube portion,                        are rigidly connected by a third node plate in the vicinity of a location                    where the respective outlet portion passes into the respective outlet bend, and                        are rigidly connected by a fourth node plate in the vicintiy of a location                    where the respective outlet bend passes into the respective second straight tube portion.                        
According to a first development of the invention and/or of the above preferred embodiment, electrodynamic velocity sensors are used and the excitation system is of the electrodynamic type.
According to a second development of the invention, which can also be used with the above preferred embodiment and/or the first development,                the support frame is of one-piece construction and is made of stainless sheet steel of constant width and thickness having a front face and a rear face, comprises:                    a plane inlet frame portion, which has the inlet manifold welded therein,            a plane outlet frame portion, which has the outlet manifold welded therein,            a plane feedthrough frame portion connecting the inlet frame portion and outlet frame portion and having the feedthrough mounted therein in a pressure-tight manner,            a first plane extension frame portion extending from the inlet frame portion at an angle greater than 90°,            a bent vertex frame portion passing into the first extension frame portion, and            a second plane extension frame portion extending from the outlet frame portion at said angle and passing into the vertex frame portion; and                        the support frame is supplemented by a plane front sheet of stainless steel, which is welded to the front, and a plane rear sheet of the same steel, which is welded to the rear face, to form the housing.        
According to a third development of the invention, which can also be used with the preferred embodiment and/or the first and/or second developments, the feedthrough comprises:                a flange attached to the support frame and having a hole;        the printed-circuit board, which is passed through a slot formed in the feedthrough frame portion and extends into the flange, with the printed-circuit board and the slot separated by a distance sufficient for electric isolation;        a disk of insulating material resting on the feedthrough frame portion and through which the printed-circuit board is passed; and        an insulating compound filling a portion of the hole lying above the disk, the insulating compound having a thickness at least equal to the gap length specified for type of protection Ex-d as a function of gap width.        
One advantage of the invention is that it permits the construction of Coriolis mass flow rate/density/viscosity sensors whose overall length, i.e., the length along the inlet/outlet axis, is considerably shorter than the overall length of the assembly according to U.S. Pat. No. 5,796,011. This is due to, among other things, the V shape of the measuring tube. A compact sensor with the desired measurement accuracy is obtained.
Furthermore, the design of the housing, which consists essentially of a support frame, a front steel sheet, and a rear steel sheet, contributes to the fact that the Coriolis sensor can be manufactured at very bw cost. Manufacturing costs are also kept low through the use of the printed-circuit board for the feedthrough, since the board provides a simple and low-cost electrical connection between the excitation system and the sensors on the one hand and evaluation electronics on the other.